Synaptic transistor learns as it switches
The essential ingredient of the transistor is use of samarium nickelate (SmNiO3), which is a correlated electron system with an insulator–metal transition temperature at 130 degrees C in bulk form.
In biological synapses calcium ions and receptor sites effect a change in the synapse response. The more the synapse is fired the stronger the response becomes and this is an essential part of the way neurological systems learn. Harvard’s artificial version achieves the same plasticity with oxygen ions. When a voltage is applied, these ions slip in and out of the crystal lattice of an 80nm thick film of samarium nickelate, which acts as the synapse channel between platinum "axon" and "dendrite" terminals. The varying concentration of ions in the nickelate raises or lowers its conductance and, just as in a natural synapse, the strength of the connection depends on the time delay in the electrical signal.
Structurally, the device consists of the nickelate semiconductor sandwiched between two platinum electrodes and adjacent to a small pocket of ionic liquid. An external circuit converts the time delay into a voltage applied to the ionic liquid that either drives ions into the nickelate or removes them. The device is embedded in a silicon chip.
One of the advantages of the synapse-like transistor is that it can be operated in analog mode, just as biological synapses do. While it may be possible to simulate synapse operation using digital signal processing in a conventional CMOS process it consumes a lot of power. The human mind with approaching 100 billion neurons runs on about 20 watts of energy.
The synaptic transistor also offers the advantage of non-volatile memory, which means it remembers its state when power is removed, which is another key to saving power.
"There’s extraordinary interest in building energy-efficient electronics these days," said principal investigator Shriram Ramanathan, associate professor of materials science at Harvard SEAS. "Historically, people have been focused on speed, but with speed comes the penalty of power dissipation. With electronics becoming more and more powerful and ubiquitous, you could have a huge impact by cutting down the amount of energy they consume."
The extreme sensitivity of electrical properties to defects in correlated oxides may make them a particularly suitable class of materials to realize artificial biological circuits that can be operated at and above room temperature and integrated into conventional electronic circuits.
"In this paper, we demonstrate high-temperature operation, but the beauty of this type of a device is that the learning behavior is more or less temperature insensitive, and that’s a big advantage," said Ramanathan. "We can operate this anywhere from about room temperature up to at least 160 degrees Celsius."
The synapse performance in this proof-of-concept device is partially dependent on physical scale. Ramanathan and his research team are planning, along with microfluidics experts at Harvard, to investigate limits of miniaturization and performance.
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